* European soil moisture map https://tmsnrt.rs/2M4kIvA
OSLO, Aug 1 (Reuters) - This year's unusually warm summer in the Nordic region has increased sea water temperatures and forced some nuclear reactors to curb power output or shut down altogether, with more expected to follow suit.
The summer has been 6-10 degrees Celsius above the seasonal average so far and has depleted the region's hydropower reservoirs, driving power prices to record highs, boosting energy imports from continental Europe and driving up consumer energy bills.
Nuclear plants in Sweden and Finland are the region's second largest power source after hydropower dams and have a combined capacity of 11.4 gigawatts (GW).
Reactors need cold sea water for cooling but when the temperature gets too high it can make the water too warm for safe operations, although the threshold varies depending on the reactor type and age.
Unscheduled power output cuts in Swedish and Finnish reactors could push prices even higher, said Vegard Willumsen, section manager at Norway's energy regulator NVE.
"If nuclear reactors in the Nordics shut down or reduce power due to the heatwave, it could also put pressure on the supply and consequently on the Nordic power prices," he added.
WHY IS WATER TEMPERATURE AN ISSUE?
The Nordic region's nuclear plants comprise either pressurized water reactors (PWR) or boiling water reactors (BWR) - and both can be affected by warm sea water.
Typically, power would be reduced at the 12 reactors after a certain temperature threshold has been reached and then fully shut down at a higher threshold.
BWRs can keep operating for longer and would only shut down after a several-degree rise in water temperatures from the moment power reductions are triggered.
However, PWRs require a shorter time to shut down after they start reducing power.
Utility Vattenfall, which operates seven reactors in Sweden, shut a 900 megawatt (MW) PWR unit - one of the four located at its Ringhals plant - this week as water temperatures exceeded 25 degrees Celsius.
The firm's second plant at Forsmark consists of three BWRs and Vattenfall had to reduce output by 30-40 megawatt per reactor earlier in July as the sea water in the area exceeded 23 degrees Celsius.
Finland's Fortum reduced power at its Loviisa plant last week when water temperatures reached 32 degrees C, close to a threshold of 34 degrees.
The extent to which water temperature affects nuclear plants also depends on the depth that they receive water from. Colder water is deeper.
It also depends on how warm the water is after being used in the reactors and released back into the sea. If used water exceeds 34 degrees Celsius, it can cause major output reductions or shutdowns for certain plants due to safety regulations.
Sweden's biggest reactor - 1.4 GW Oskarshamn 3 - should be less vulnerable to very hot summers due to the depth of water, said a spokesman for operator OKG, a unit of Uniper Energy .
"Water intake (is) at a depth of 18 meters where the water naturally is cooler than on the surface ... should it be too hot, we would of course reduce the capacity accordingly," he said.
Oskarshamn 3 will reduce power if sea water reaches 25 degrees but it was below 20 degrees on Tuesday.
Similarly, Teollisuuden Voima's Olkiluoto plant in Finland has deeper water which is colder than a 27-degree threshold.
TVO has also built an additional safety mechanism - a canal - which it can use under certain conditions to release used warm water at the other side of the Olkiluoto island.
Daily sea water temperatures for the North and Baltic Seas can be found mapped by the Danish Meteorological Institute at the following link: https://www.dmi.dk/en/hav/satellitmaalinger/havtemperatur
(Editing by Nina Chestney/David Evans)